I have had the 4E Underdark source book on preorder for a while. I noticed this afternoon that the local B&N had one in stock, which I thought was strange since it does not officially release until Tuesday. I did not flip around through the book very much; was more or less pressed for time and somewhat distracted keeping up with 3 kids!
I also noticed that there was a single copy of the 4E Player’s Handbook Races: Dragonborn supplement, but right now I am not interested in these types of supplements. Other than trying to make money for WotC, I am not sure what purpose these race supplements serve. The only reason that I would pick it up is for a brief reading (it is all of 32 pages) and to flip it a few years down the road, which assumes it will become a rare and/or high in demand book. Besides, the PHB3 should be out later this spring; seem like this sort of material is best served with the PHBx books.